For less than £8bn a year - about as much as humans spend on ocean cruises - governments could protect declining world stocks of fish in a series of marine nature reserves around the globe, British scientists report today.

Andrew Balmford of Cambridge University, Callum Roberts of the University of York and colleagues report in the Proceedings of the National Academy of Sciences that that figure would be enough to protect 30% of the oceans from all fishing. Such an investment could create more than 1 million jobs and would safeguard, and eventually increase, a global fish catch worth up to £44bn a year.

"Our study suggests that we could afford to conserve the seas and their resources in perpetuity, and for less than we're now spending on subsidies to exploit them unsustainably," Dr Balmford said.

Prof Roberts said: "We have barely even begun the task of creating marine parks. Here in Britain, a paltry one fiftieth of one percent of our seas is encompassed by marine nature reserves and only one fiftieth of their combined area is closed to fishing. Yet the seas are being devastated by unsustainable fishing, pollution and mineral exploitation."

The researchers say that the world's oceans are in trouble: global fish catches are in decline, populations of whales, dolphins, sea otters and other marine mammals have collapsed and habitats - reefs, estuaries and other submarine landscapes vital for breeding - have been damaged or destroyed.

Coastal states pledged at a world summit in 2002 to create national networks of marine parks, and a congress in Durban last year recommended that at least 20% to 30% of every marine habitat should be protected from fishing.

This would mean closing parts of traditional fishing grounds off Iceland, or the Grand Banks or the Barents Sea, as well as estuaries, mangrove swamps and coral reefs closer to land. "If you put areas off limits to fishing, there is no more effective way of allowing things to live longer, grow larger and produce more offspring," Prof Roberts said.

Dr Balmford said: "Meeting this commitment to marine protection will require international effort on an unprecedented scale. Just half a percent of the sea lies in marine parks today, compared with 12% of the land."

The scientists surveyed the running costs of 83 existing parks they judged to be well-managed, and found that most felt they needed twice the income to do the job properly. From this, they calculated that nearly £8bn a year could cover the cost of protecting 30% of the world's seas: 20% coverage could be achieved for just over £5bn a year.

This would not only protect global fish stocks; it would underwrite the sustained delivery of what biologists call "ecosystem services" to humanity valued at £385bn a year. Coral reefs protect tropical shorelines from erosion, creatures in coastal seas and estuaries scavenge and decompose epic quantities of human waste, and marine plankton provide most of the planet's oxygen.

Furthermore, such a marine park programme would cost less than the estimated £8bn to £16bn a year spent on economically and environmentally damaging subsidies to commercial fisheries.

Prof Roberts said: "If you could drain the North Sea, what you would see would be something like the endless muddy fields of East Anglia with not a meadow, copse or forest in sight. Almost all of the seabed is put to the trawl because there are virtually no protected areas to offer refuge to vulnerable animals like skates, sharks, corals and seafans."